Objective Left heart underdevelopment is commonly observed in fetuses with left diaphragmatic hernia. This finding has been attributed to compression of the left atrium by herniated abdominal organs, redistribution of fetal cardiac output and/or low pulmonary venous return. As preferential right or left heart underdevelopment is usually not a feature of right diaphragmatic hernia, we searched for an alternative mechanism. Since in normal fetuses the major fraction of left heart filling is provided by the ductus venosus via the inferior caval vein and oval foramen, our study focused in particular on the streaming direction of these structures.

Patients and methods We prospectively studied 32 fetuses with left diaphragmatic hernia between 19+6 weeks and 38+6 weeks of gestation by echocardiography. The fetuses were divided into two groups: Group I fetuses exhibited abnormal streaming of ductus venosus and inferior caval vein blood flow towards the right side of the heart; group II fetuses did not exhibit this abnormal flow direction. Cardiac inflow and outflow dimensions were compared in the two groups.

Results 18 of 19 group I fetuses with left diaphragmatic hernia exhibited disproportionately smaller left than right heart dimensions; 12 of 13 group II fetuses exhibited similar sized left and right cardiac inflow and outflow dimensions (p<0.05).

Conclusions Preferential ductus venosus and inferior caval vein streaming towards the fetal right heart offers another haemodynamic mechanism for left heart underdevelopment in fetuses with left diaphragmatic hernia. The pathoanatomical basis of this abnormal flow pattern results from intrathoracic abdominal organ herniation and rightward displacement of the heart.